Based on the various flashbacks, Ethan was a known 'bully' of the MC, who hated the very close relationship Erynn and MC had. There's a lot of indications in the flashbacks, and from Erynn herself, that this was not the first time she had problems with the Ethan and the MC. So staying back to argue *yet again* would accomplish what? The message hadn't sunk in to that point, why would it yet again?
For that matter, regardless of timing of the confession, why would she leave MC to go listen to Ethan talk about a social media post? Really? That's an appropriate time for that, since she liked the MC? Even if she was embarrassed, there's no way to say that's blameless.
Lexi left Erynn and Ethan and must have "forced" herself into the MCs space when he was sitting outside then too. She even had a conversation with him, kissed him on the cheek, and wished him happy birthday. Perhaps if Erynn had, instead of arguing yet again with Ethan, gone after MC she could have had some "forced" time instead of Lexi to talk about it. But no, she wanted to stay and argue with Ethan, yet again.
So no, Erynn was not entirely blameless. Taking all of the flashback details, and MC's confrontation with Erynn about his confession, into account. MC is also at fault for being stubborn-headed and not actually letting people talk, and making wild assumptions, as seen throughout the entire story to-date.
